I was exactly in your shoes a few months ago when I decided I wanted more ponies than just the ones I had. I think the best way to get started is just to educate yourself on what is out there first. I had only G1 ponies from when I was a kid and I knew about the G3's because I had seen them in the store. I really had no grasp about how many kinds and how many of each kind when I got started. I STILL find myself seeing ponies in other people's collections I did not know existed. So just take some time to look at what's out there and see what it is you like. Here's some great resource links.
Dream Valley:
Dream Valley - A Collector's Guide to My Little Pony
Strawberry Reef:
G1, G2, G3 & G4 My Little Pony Reference Guide
The board offers great features you help you out. There's the Wishing Well where you can just keep a personal list of the ponies you'd like to own and you can access it and edit it at any time. I'm constantly adding and taking off ponies as I change my mind about what I want.
I highly suggest not going crazy at first and thinking you need one of every pony in existence. I noticed my Wishing Well list was massive when I first started and then I decided there were some ponies I just didn't need and I took them off. Or some were purchased and were removed as well.
Now, as for obtaining said ponies, the board offers great features as well. If you have money set aside for certain ponies, you can post on the "Want to Buy" section that you are looking for certain ponies and it's possible people will contact you with a pony they'd like to sell.
The best way to get ponies is to lurk around the "Want to sell" section and see what's around. That will help you get an idea for how much the ponies are going for so you can keep tabs on maybe how much you would have to spend for a certain pony on your list.
Oh man, it is an evil, evil place. Sometimes I go there just to see what's up and I ALWAYS see something I totally want and sometimes it's hard to not inquire when i don't have the funds. It's best not to go there unless you are ready to buy something.
If you just want cheap, random ponies, I suggest trying your local thrift stores, yard sales and flea markets. They are super cheap if you don't mind sometimes cleaning them up. I LOVE it when people take pictures of their finds from those places in the "Show and Tell" section. So much fun what people happen upon.
I hope that helps you out, hon. Good luck with your collection!